Fast excitatory synaptic transmission in the CNS is mediated by AMPA-type glutamate receptor (GluR) channels. Heterologous expression suggested that the Ca2+ permeability of these receptors critically depends on the subunit composition. Using patch-clamp techniques in brain slices, we found that the Ca2+ permeability of native AMPA-type GluRs was markedly higher in nonpyramidal (PCa/PK approximately 0.63) than in pyramidal (PCa/PK approximately 0.05) neurons of rat neocortex. Analysis of mRNA in single cells indicated that the relative abundance of GluR-B-specific mRNA was significantly lower in nonpyramidal (GluR-B/GluR-non-B approximately 0.3) than in pyramidal (GluR-B/GluR-non-B approximately 3) cells. This suggests that differences in relative abundance of GluR-B-specific mRNA generate functional diversity of AMPA-type GluRs in neurons with respect to Ca2+ permeability.